Glass composition and substrate for plasma display

ABSTRACT

A glass composition comprising from 45 to 66 wt % of SiO 2 , from 0 to 15 wt % of Al 2  O 3 , from 10 to 24 wt % of Li 2  O+Na 2  O+K 2  O, from 14 to 26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % of SO 3  +Sb 2  O 3 , said glass composition containing substantially no zirconia and having a strain point of at least 560° C. and a linear thermal expansion coefficient of at least 80×10 -7  /°C. within a temperature range of from 50° to 350° C.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a glass composition suitable forsubstrates for display devices such as fluorescent character displaytubes, plasma display panels, flat cathode ray tubes and liquid crystaldisplay tubes.

As a glass substrate for plasma display, a soda lime glass having astrain point of about 510° C., formed by float process, is commonlyused. A typical plasma display panel is produced by the followingprocess.

Firstly, on a glass substrate for the display surface side, displayelectrodes will be printed, and dielectric layer will be printedthereon, followed by baking. Further, a protective film will bevapor-deposited on this dielectric layer. On the other hand, on theopposing rear-side glass substrate, Al, Ag or Ni electrodes, andstripe-shaped partition walls (low melting point glass) to preventelectric discharge between electrodes and to prevent color mixing ofred-, green- and blue-phosphors, will be formed by baking at atemperature of from 500° to 600° C. Further, printed circuits will beformed, and red-, green- and blue-phosphors will be printed.

The display side and rear side glass substrates will be bonded by meansof a low melting temperature glass frit within the same temperaturerange as the above-mentioned temperature, and a gas mixture of xenon andneon as the main discharge gas, will be sealed in to obtain a plasmadisplay panel.

The glass substrate for plasma display is subjected to heat treatment ata temperature equal to or higher than the strain point of soda limeglass at a level of from 500° to 600° C., and thermal deformation islikely to take place. Therefore, when soda lime glass substrates areused, a 40 inch panel is almost at the limit, and it is substantiallydifficult to use soda lime glass substrates for high definition TV whichrequires a panel of a larger size with a high level of resolution.

A ZrO₂ -containing glass is also known which undergoes a less degree ofdeformation by such heat treatment (Japanese Unexamined PatentPublication No. 40933/1991). However, this glass is susceptible toscratching, and it is necessary not to polish the glass or to polishwith a due care not to form a substantial scratch mark. Further, evenwith a glass having no scratch mark, a scratch mark may form during theproduction process. Therefore, a due care will be required for handlingin the process.

Each of these methods reduces the amount of glass substrate which can beproduced per unit period of time. In other words, each method has aproblem that the production cost of glass increases.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a glass compositionwhich scarcely undergoes thermal deformation and which is hardlysusceptible to scratching.

That is, the present invention provides a glass composition comprisingfrom 45 to 66 wt % of SiO₂, from 0 to 15 wt % of Al₂ O₃, from 10 to 24wt % of Li₂ O+Na₂ O+K₂ O, from 14 to 26 wt % of CaO+MgO+SrO+BaO+ZnO, andfrom 0 to 1 wt % of SO₃ +Sb₂ O₃, said glass composition containingsubstantially no zirconia and having a strain point of at least 560° C.and a linear thermal expansion coefficient of at least 80×10⁻⁷ /°C.within a temperature range of from 50° to 350° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the reason for the definition of the composition of the glasscomposition of the present invention will be described.

In the present invention, if the glass composition contains ZrO₂substantially, the glass tends to be susceptible to scratching, and thebreakage frequency during the production process tends to be high,whereby the production yield will be poor. Therefore, the glasscomposition of the present invention contains substantially no zirconia.Here, "contains substantially no zirconia" means that the zirconiacontent is less than 0.2 wt %.

Further, if the glass composition contains As₂ O₃ substantially, arsenicwill dissolve in the polishing waste liquid resulting from polishing inthe commercial production, whereby a substantial cost will be requiredfor treatment of the waste liquid. Therefore, the glass compositionpreferably contains substantially no arsenic. Here, "containssubstantially no arsenic" means that the As₂ O₃ content is less than 0.1wt %.

Further, if the glass composition contains B₂ O₃ substantially, thelinear expansion coefficient tends to be small, and boron tends toevaporate during the production process, whereby it tends to bedifficult to produce a uniform glass. Further, evaporated boron islikely to erode the bricks of the furnace. Accordingly, it is preferredthat the composition does not substantially contain B₂ O₃. Here, "doesnot substantially contain B₂ O₃ " means that the content is less than0.1 wt %.

If the linear thermal expansion coefficient is less than 80×10⁻⁷ /°C.,within a temperature range of from 50° to 350° C., cracks are likely toform in the electrodes and the partition walls. Therefore, the linearthermal expansion coefficient is at least 80×10⁻⁷ /°C., preferably atleast 85×10⁻⁷ /°C. Further, the linear thermal expansion coefficient ispreferably less than 120×10⁻⁷ /°C., more preferably less than 95×10⁻⁷/°C.

If the strain point is less than 560° C., thermal deformation is likelyto form during the heat treatment of e.g. electrodes and partitionwalls. Therefore, the strain point is usually at least 560° C.,preferably at least 570° C.

SiO₂ is a main component for the glass. If the content of SiO₂ is lessthan 45 wt %, the chemical durability tends to be poor, particularlycorrosion by hydrofluoric acid tends to be substantial. On the otherhand, if it exceeds 66 wt %, the linear thermal expansion coefficientwill be less than 80×⁻⁷ /°C. Therefore, the SiO₂ content is from 45 to66 wt %. It is preferably within a range of from 50 to 63 wt % in orderto obtain a higher strain point (at least 570° C.).

Al₂ O₃ which may optionally be used, is a component which is effectiveto increase the strain point without substantially reducing the linearthermal expansion coefficient. If the content of Al₂ O₃ exceeds 15 wt %,the liquidus temperature tends to be too high. Therefore, the content ofAl₂ O₃ is usually within a range of from 0 to 15 wt %, preferably from0.5 to 12 wt %, more preferably from 5.5 to 10 wt %.

Li₂ O, Na₂ O and K₂ O are components which are effective for adjustingthe linear thermal expansion coefficient of glass or the viscosity at ahigh temperature. These components may not all be incorporatedsimultaneously. However, if their total content is less than 10 wt %,the linear thermal expansion coefficient tends to be less than 80×10⁻⁷/°C., and if it exceeds 24 wt %, it becomes difficult to bring thestrain point to a level of at least 560° C. Therefore, their totalcontent is within a range of from 10 to 24 wt %. The content of Na₂ O ispreferably from 0 to 6 wt %, and the content of K₂ O is preferably from0 to 0.5 wt %.

Among them, K₂ O has an effect of increasing the strain point, and itscontent is preferably from 4 to 20 wt %, particularly from 9 to 16 wt %.

SrO, BaO, ZnO, CaO and MgO may not all be incorporated simultaneously.However, if their total content is less than 14 wt %, it tends to bedifficult to bring the strain point to a level of at least 560° C. Onthe other hand, if it exceeds 26 wt %, the strain point tends to be toolow. Therefore, their total content is usually within a range of from 14to 26 wt %, preferably from 17 to 23 wt %, more preferably from 18 to 20wt %.

The content of CaO is preferably from 0 to 14 wt %, the content of MgOis preferably from 0 to 6 wt %, the content of BaO is preferably from 0to 14 wt %, and the content of ZnO is preferably from 0 to 6 wt %.

Among them, SrO has an effect of increasing the strain point, and itscontent is preferably from 1 to 14 wt %, particularly from 4 to 10 wt %.Further, CaO is preferably contained in an amount of from 1 to 14 wt %.

Sb₂ O₃ and SO₃ which may optionally be incorporated, are refiningagents. If their total content exceeds 1 wt %, their effects of refiningwill be saturated. Therefore, their total content is within a range offrom 0 to 1 wt %.

In addition to the above components, the following components may beincorporated.

CeO₂ may be incorporated for the purpose of suppressing browning whichis likely to result when the glass is irradiated with X-rays. TiO₂ andBi₂ O₃ may be incorporated for the purpose of suppressing solarizationwhich is likely to result when the glass is irradiated with ultravioletrays. PbO has an effect of suppressing solarization, but even if it isadded excessively, the effect will be saturated. Therefore, the contentis preferably less than 0.3 wt %, more preferably less than 0.1 wt %.Further, the color of glass can be controlled by incorporating a smallamount of Fe₂ O₃, CoO, Cr₂ O₃ or NiO.

On the other hand, it is preferred that fluorine, zirconium, boron,phosphorus and arsenic are not substantially contained, although theymay be incorporated as impurities.

The glass composition of the present invention can be produced bysupplying glass batch formulated to have the desired composition, into amelting furnace, followed by vitrification and forming into atransparent and not substantially crystallized sheet glass having apredetermined thickness by float process.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted by such specific Examples.

EXAMPLES 1 to 6 and COMPARATIVE EXAMPLES 7 to 9

550 g of glass batch formulated to have the desired composition, wereput into a platinum crucible and heated for 4.5 hours for vitrificationin a furnace of 1510° C. while stirring from time to time. Then, themolten glass was cast in a graphite mold and then annealed to reducestrain. Table 1 shows the composition by weight % of oxides. The amountsof CoO and NiO were so small that they were represented by ppm.

With respect to such a sheet glass, the linear thermal expansioncoefficient, the strain point, the brittleness, the temperature at 10²poise, which is an index for the solubility, the temperature at 10⁴poise, which is an index for formability, the liquidus temperature, thechemical durability and the electrical resistance were measured, and theresults are shown in the respective columns in Table 1. Table 1 alsoshows Comparative Examples.

Various properties were measured as follows.

The brittleness was determined in such a manner that a Vickersindentator was pressed against a mirror-polished glass surface under aload of 500 g, whereupon the diagonal length (a) of the indentation andthe crack length (C) were measured, and their ratio C/a was obtained.The larger the ratio C/a, the higher the brittleness [Sehgal et al. J.Mat. Sci. letters [14], p.167-169, 1995].

The chemical durability was represented by the weight reduction per unitarea (mg/cm²) when a test sample of 50 mm×50 mm×3 mmt was immersed in a15 wt % HF aqueous solution at 40° C. for 90 seconds.

The linear thermal expansion coefficient is represented by a valuewithin a temperature range of from 50° to 350° C., and the electricalresistance is represented by a logarithmic value of the resistance(unit: Ω·cm) at 150° C. Other physical properties were measured inaccordance with methods which are commonly employed in the glassindustry.

The liquidus temperatures of all glasses in Examples and ComparativeExamples in Table 1, are all lower than the temperatures of therespective glasses at 10⁴ poise. This indicates that they can be formedby float process. The ratio C/a of the soda lime glass in ComparativeExample 9 is lower than C/a of the glasses in Examples 1 to 8, whichindicates that the strain point is low as compared with that of theglasses in the Examples, and such soda lime glass can not be used for aplasma display substrate of a large size with high resolution, althoughit is scarcely brittle. The glass in Comparative Example 10 isequivalent to the glasses in Examples 1 to 8 with respect to both thestrain point and the linear expansion coefficient, and the ratio C/a isalso small, which indicates that the glass is scarcely brittle. However,the temperature at 10² poise is higher by as much as 200° C. than thoseof the glasses of Examples 1 to 8, which indicates that melting is verydifficult. The ratio C/a of the glass in Comparative Example 11 whichcontains zirconia, is higher than the ratios C/a of the glasses inExamples 1 to 8, which indicates that it is readily brittle.

As shown by the foregoing Examples, the glass composition of the presentinvention has a high linear thermal expansion coefficient and a highstrain point, and it can relatively easily be melted and scarcelybrittle.

                                      TABLE 1                                     __________________________________________________________________________                                                Comparative                                   Examples                        Examples                          Composition 1   2   3   4   5   6   7   8   9   10  11                        __________________________________________________________________________    SiO.sub.2   64.4                                                                              57.2                                                                              61.2                                                                              62.4                                                                              53.8                                                                              62.6                                                                              46  64.1                                                                              72.3                                                                              62.4                                                                              57.5                      Al.sub.2 O.sub.3                                                                          1   5.6 5.7 6.1 5.6 6.1 15  4.2 2   17.1                                                                              7                         Li.sub.2 O  0   0   0   0   0   0.2 0   0.2 0   0   0                         Na.sub.2 O  0   0   0   0   1.0 0   3.2 1.4 12.5                                                                              12.2                                                                              0                         K.sub.2 O   11  10.6                                                                              13.4                                                                              11.6                                                                              9.4 11.6                                                                              19.1                                                                              10  1   3.8 4.3                       SrO         13  5   5.8 9.2 4.3 9.2 6.7 0   0   0   6                         BaO         0.4 12.8                                                                              0   0   11.4                                                                              0   0   7   0   0   9                         ZnO         5   0   2.7 0.3 10.2                                                                              0.3 0   0   0   0   4                         CaO         0   3.4 8.1 10  1.4 10.0                                                                              3.6 7.7 8   0.5 0                         MgO         5   5   2.8 0   2.9 0   5   5   4   3.5 7                         ZrO.sub.2   0   0   0   0   0   0   0   0   0   0   2                         CeO.sub.2   0.5 0   0   0   0   0   0   0   0   0   3                         TiO.sub.2   0.4 0   0.1 0   0   0   0   0   0   0   0                         PbO         0   0.2 0   0   0   0   0   0   0   0   0                         Bi.sub.2 O.sub.3                                                                          0   0   0   0   0   0   0.3 0   0   0   0                         Fe.sub.2 O.sub.3                                                                          0   0   0   0   0   0   0.2 0.1 0   0   0                         CoO (ppm)   18  0   0   0   0   0   0   0   0   0   0                         NiO (ppm)   140 0   0   0   0   0   0   0   0   0   0                         SO.sub.3    0.1 0.2 0.2 0   0   0   0.4 0   0.2 0   0                         Sb.sub.2 O.sub.3                                                                          0.2 0   0   0.4 0   0   0.5 0.3 0   0.5 0.2                       Linear expansion                                                                          82  85  85  82  83  83  116 82  85  88  82                        coefficient (× 10.sup.-7 /°C.)                                   Strain point (°C.)                                                                 567 587 587 594 571 586 570 572 511 576 589                       Brittleness 2.61                                                                              2.97                                                                              2.62                                                                              2.64                                                                              2.91                                                                              2.65                                                                              3.07                                                                              2.63                                                                              2.43                                                                              2.43                                                                              3.16                      Temp. at 10.sup.2 poise                                                                   1565                                                                              1560                                                                              1560                                                                              1565                                                                              1549                                                                              1550                                                                              1518                                                                              1564                                                                              1470                                                                              1770                                                                              1504                      Temp. at 10.sup.4 poise                                                                   1145                                                                              1161                                                                              1161                                                                              1155                                                                              1169                                                                              1143                                                                              1118                                                                              1165                                                                              1044                                                                              1270                                                                              1155                      Liquidus temp. (°C.)                                                               1125                                                                              1110                                                                              1110                                                                              1107                                                                              992 1107                                                                              1042                                                                              1112                                                                              1040                                                                              1230                                                                              1150                      Chemical durability                                                                       2.2 4.3 3.4 3.3 6.9 3.3 10  0.8 4.2 3.8 5.2                       Electrical resistance                                                                     11.4                                                                              11.7                                                                              11.3                                                                              11.5                                                                              12.4                                                                              11.5                                                                              11.6                                                                              11  8.8 8.4 10.9                      __________________________________________________________________________

The glass composition of the present invention is scarcely brittle andhas a strain point of at least 560° C. and a linear thermal expansioncoefficient close to that of soda lime glass, whereby it can be used foran application where a high strain point is required among glasssubstrates for which soda lime glass has heretofore been used. It isparticularly useful for a plasma display substrate, but it is alsouseful for other substrates such as substrates for e.g. liquid crystaldevices. Further, the glass composition of the present invention has ahigh electrical resistance of at least 10¹¹ Ω·cm at 150° C., and it isaccordingly useful for an application where electrical insulation isparticularly required.

What is claimed is:
 1. A glass composition comprising from 50 to 66 wt %of SiO₂, from 0 to 15 wt % of Al₂ O₃, from 0 to 13.4 wt % of K₂ O, from10 to 24 wt % of Li₂ O+Na₂ O+K₂ O, from 0 to 7 wt % of BaO, from 14 to26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % of SO₃ +Sb₂ O₃,said glass composition containing substantially no zirconia and having astrain point of at least 560° C. and a linear thermal expansioncoefficient of at least 80×10⁷ /°C. within a temperature range of from50° to 350° C.
 2. The glass composition according to claim 1, which hasa linear thermal expansion coefficient of from 80×10⁻⁷ to 120×10⁻⁷ /°C.within a temperature range of from 50° to 350° C.
 3. The glasscomposition according to claim 1, which has a linear thermal expansioncoefficient of from 80×10⁻⁷ to 95×10⁻⁷ /°C. within a temperature rangeof from 50° to 350° C.
 4. The glass composition according to claim 1,wherein the liquidus temperature is lower than the temperature at whichthe viscosity is 10⁴ poise.
 5. The glass composition according to claim1, which contains substantially no arsenic.
 6. The glass compositionaccording to claim 1, which comprises from 50 to 63 wt % of SiO₂, from5.5 to 15 wt % of Al₂ O₃, from 0 to 6 wt % of Na₂ O, from 1 to 13.4 wt %of K₂ O, from 0 to 0.5 wt % of Li₂ O, from 10 to 24 wt % of Li₂ O+Na₂O+K₂ O, from 0 to 14 wt % of CaO, from 0 to 6 wt % of MgO, from 1 to 14wt % of SrO, from 0 to 7 wt % of BaO, from 0 to 6 wt % of ZnO, from 14to 26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % of SO₃ +Sb₂ O₃,and which contains substantially no zirconia.
 7. The glass compositionaccording to claim 6, which has a strain point of at least 570° C.
 8. Asubstrate for plasma display which is made of a glass compositioncomprising from 50 to 66 wt % of SiO₂, from 0 to 15 wt % of Al₂ O₃, from0 to 13.4 wt % K₂ O, from 10 to 24 wt % Li₂ O+Na₂ O+K₂ O, from 0 to 7 wt% of BaO, from 14 to 26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt% of SO₃ +Sb₂ O₃, said glass composition containing substantially nozirconia and having a strain point of at least 560° C. and a linearthermal expansion coefficient of at least 80×10⁻⁷ /°C. within atemperature range of from 50° to 350° C.
 9. A substrate for plasmadisplay which is made of a glass composition comprising from 50 to 63 wt% of SiO₂, from 5.5 to 15 wt % of Al₂ O₃, from 0 to 6 wt % of Na₂ O,from 1 to 13.4 wt % K₂ O, from 0 to 0.5 wt % Li₂ O, from 10 to 24 wt %Li₂ O+Na₂ O+K₂ O, from 0 to 14 wt % of CaO, from 0 to 6 wt % of MgO,from 1 to 14 wt % of SrO, from 0 to 7 wt % of BaO, from 0 to 6 wt % ofZnO, from 14 to 26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % ofSO₃ +Sb₂ O₃ and which contains substantially no zirconia.